Size the tank from the largest swing in the running balance between water pumped in and water drawn out. Start with hourly data for a representative day in the critical irrigation month. Then model low-sun days and add any reserve needed for the project’s service target.
In a simplified example, the irrigation system uses 8 m³ before pumping starts. During the pumping window, the pump delivers 20 m³ while irrigation uses 4 m³. Irrigation draws a further 8 m³ after pumping stops. The cumulative balance relative to the start of the day moves from 0 to −8, then to +8, and finally back to 0. The swing is 16 m³. This schedule therefore needs at least 16 m³ of usable balancing volume under the stated assumptions, with enough water in the tank at the start of the day.
Check the supplier’s volume between the actual low and high operating levels. Allow separately for unusable volume below the outlet and any reserve needed for cloudy weather or interruptions. If the daily water balance remains negative for several days, the tank can cover the shortfall only until the reserve is exhausted. Check the expected pump delivery at actual head, available solar energy and the water source’s sustainable yield before specifying more storage.
Send the hourly irrigation schedule, pump and motor data, design head, PV module sheet and tank level signals with your inquiry. For pump drive selection, use the solar drip irrigation guide and the S80 pump inverter page.
References
Practical Action · Solar Pumping for Water Supply，§5.3.7 · FAO · Water Lifting Devices